Past and present highlights of ROTA research

G.E. Volovik and V.P. Mineev present a general topological classification of different structures in the 3He superfluids.
Construction of a rotating nuclear refrigeration cryostat, capable of cooling liquid 3He down to 0.3 mK, is completed in the Low Temperature Laboratory. 3He-A in a rotating container is studied using nuclear magnetic resonance (NMR) techniques, and the first observations of vortices are made .
NMR measurements on 3He-B prove that the vortex core undergoes an unexpected phase transition. The two different vortices belong to the same topological class but possess cores with different structures. This observation triggers renewed theoretical efforts to investigate the symmetry and topology of the vortex core.
The singular vortex core in 3He-B is found to display a spontaneous ferromagnetic moment, manifested by the gyromagnetic effect. Theoretical investigation shows that parity is spontaneously broken inside the core .
Vortices, which had been observed in 3He-A, are identified as continuous structures without a singular core and with n = 2 quanta of superfluid circulation .
Indications of two different vortex structures in 3He-A are obtained from measurements on the propagation of negative ions through rotating superfluid.
The nature of the vortex core transition in 3He-B is explained. On the low temperature side, the core splits into two so that axial symmetry is broken. In contrast, the high temperature vortex is axisymmetric.
Vortex-free counterflow is found to exist in 3He-B at low W . Later, vortex nucleation is observed to be suppressed in a new smooth-walled container to so high speeds of rotation that a hierarchy of four different vortex-free textures as a function of W can be measured .
Construction of the second rotating cryostat in the LTL is completed.
Continuous vortices are observed to exist in the magnetic superfluid phase 3He-A1. This is concluded from the attenuation of ultrasound.
A topological transition of continuous vortices in 3He-A is discovered from the attenuation of ultrasound as a function of the applied magnetic field. The anisotropy of 3He-A is described by two unit vectors l and d. Vortices are characterized by two topological charges, nl and nd, which determine how many times l and d sweep around the unit sphere while one moves across the vortex. When the field is in-creased, nd drops discontinuously from 1 to 0, while nl = 1 remains constant.
Absorption from coherent spin precession gives direct experimental evidence for the broken axial symmetry of the 3He-B vortex core at low temperatures.
Discovery is made of a new phase transition associated with continuous vortices in 3He-A under rapid rotation in a low magnetic field. One current interpretation explains it in terms of a topological phase transition within a layer of counter-vortices near the walls of the rotating container. Even a state with continuous vortices is not completely devoid of singularities: at the top and bottom ends the vortices terminate in point singularities, so-called boojums. These defects can be avoided if vortices form closed loops, with the returning section near the surface. In this case the superfluid component takes part in solid-body like rotation except in the surface layer, where the superfluid is at rest.
Extensive measurements are started on the critical velocity of vortex creation. It is found that the nucleation threshold is an order of magnitude lower for continuous vortices than for singular ones.
First optical measurements are carried out on superfluid 3He. The rapid change in the viscosity at the superfluid transition is seen directly: The free surface of a thin layer of liquid reaches its equilibrium orientation, perpendicular to gravity, orders of magnitude faster in the superfluid than in the normal state. The new techniques, in which optical fibers are used to transmit light between room temperature and the cold parts of the cryostat, make possible photography at submillikelvin temperatures. The classical parabolic profile of the free surface of rotating superfluid 3He-B is detected optically.
A novel topological object is observed and identified in 3He-B using NMR. It consists of a 2-dimensional soliton terminating in a 1-dimensional spin-mass vortex. The latter in itself is also a combined object, which supports both superfluid mass and spin currents around its singular core. The soliton emanating from a spin-mass vortex may terminate on the container wall or in another spin-mass vortex. The latter alternative represents a doubly quantized (n = 2) vortex in which two spin-mass vortices are topologically confined by the soliton. Similar mechanisms of confinement have been discussed in particle physics and cosmology.
Experiments investigating vortices at the interface between the A and B superfluids are performed. A vortex layer is found to form in front of the moving A - B interface [18]. The topology for joining vortices of different structures through the interface is worked out.
The sensitivity of the NMR absorption measurement is improved sufficently so that single vortex nucleation events are observed in the B phase. With the improved NMR resolution the signature of singular A phase vortices (n = 1) is found while cooling below Tc under rotation. Vortices obtained during rotation through Tc are found to correspond to the minimum energy, calculated for a given rotation velocity and magnetic field. It was observed that singular (n = 1) and continuous (n = 2) vortices may coexist in the rotating container, forming an analogy of 2-component Coulomb plasma.
An unprecedented form of quantized vorticity, the vortex-soliton sheet, is discovered from NMR measurements on 3He-A. This macroscopic object consists of a soliton inside which Mermin-Ho vortices are confined. The sheet is attached at both ends to the side wall of the container and it folds several times to equidistant layers which uniformly fill the rotating vessel. At the vortex sheet the superflow velocity has a discontinuity. If a soliton wall, roughly aligned along the W -axis, is already present in the rotating container, then the metastable vortex-sheet state has the lowest critical velocity and becomes the preferred configuration compared to any other type of vorticity.
In summary, ROTA discoveries have been made at a steady or increasing rate, and there is no indication of exhaustion in new phenomena. Much of our earlier work on rotating superfluid 3He has been discussed in a comprehensive review article

Matti Krusius 15.12.94